Silicon carbide (SiC) is excellent in heat resistance and mechanical strength and physically and chemically stable. Therefore, silicon carbide is attracting attention as an environment-resistant semiconductor material. Also, in recent years, there have been increased demands for an SiC single crystal substrate, as a substrate for a high-frequency high-voltage resistant electronic device, etc.
In a case where electric power devices, high-frequency device, etc., are to be manufactured by using an SiC single crystal substrate, it may be usually performed in general to epitaxially grow an SiC thin film on a substrate by using a process called a chemical vapor deposition process (CVD process), or to directly implant thereinto a dopant by using an ion implantation process. In the latter case, annealing at a high temperature may be required after the implantation, and for this reason, the formation of a thin film using an epitaxial growth may be employed frequently.
Heretofore, a hollow defect called a micropipe may be present in an SiC single crystal substrate, and this defect may be carried over into the epitaxial film to be formed on the substrate, so as to deteriorate the characteristics and reliability of the resultant device. However, in recent years, along with the progress in techniques for producing an SiC single crystal substrate, the density micropipes may be decreased to almost zero, and therefore, the effect of other defects on the device are being studied. Among these, it is known that a basal plane dislocation is usually split into two partial dislocations in the SiC crystal, and a stacking fault is generated therebetween (please refer to Non-Patent Document 1). When this defect is present the inside of a device, the defect may adversely affect the reliability of a bipolar device, a Schottky barrier diode, etc. (please refer to Non-Patent Document 2). Accordingly, there are being made attempts to reduce the defect.
FIG. 1 schematically shows the behavior of a basal plane dislocation to be contained in an SiC single crystal substrate, when epitaxial growth is performed on the surface of an SiC single crystal substrate. As a result of the epitaxial growth, about 95% or more of the basal plane dislocations in the substrate may be converted into an edge dislocation. The reason for this is that, as shown in FIG. 1(a), the dislocation energy may be smaller and more stable, when the basal plane dislocation 1 in the substrate may be converted into an edge dislocation 3 so that the dislocation length becomes short, as compared with that in a case where the dislocation is carried over as such into the epitaxial film, so as to cause a basal plane dislocation 2.
FIG. 1(b) shows a case where the off-angle of the substrate is larger, and FIG. 1(c) shows a case where the off-angle of the substrate is smaller. When attention is paid to the relationship between the length of the edge dislocation 3, and the length of the basal plane dislocation 2 to be carried over into the epitaxial film, the reduction rate at which the dislocation length becomes shorter after the conversion into an edge dislocation 3 as shown in FIG. 1(c) may be higher, and a more stable condition is achieved in view of the energy. Accordingly, as the off-angle of the substrate is smaller, the rate of the conversion into an edge dislocation is higher. Further, along with the reduction in the off-angle of the substrate, the density of the basal plane dislocation itself appearing on the substrate surface may be decreased, and therefore, the basal plane dislocation to formed in the epitaxial film may be also decreased.
For this reason, from the standpoint of decreasing the basal plane dislocation in the epitaxial film, and of increasing the yield of the substrate from an SiC ingot, a substrate having an off-angle of 4° or less, which is smaller than the conventional off-angle of 8° may mainly be used at present. For example, in a case where a 4°-off substrate is used, the density of the dislocations which are not converted into edge dislocations but are carried over as basal plane dislocations into the epitaxial film, may be approximately from 100 to 200 dislocations/cm2. However, if the area of the device becomes larger, even with the basal plane dislocations to such an extent, the probability that stacking faults which have been caused by the dislocations are present inside the device may be increased, and this may give rise to a deterioration in the device characteristics or yield.
On the other hand, when the off-angle of the substrate becomes smaller, the number of steps which are present on the substrate may be decreased, and therefore, so-called step-flow growth can hardly occur at the time of the epitaxial growth. As a result, another defect such as triangular defect may be increased, to thereby pose a problem such as deterioration in the device characteristics, or reduction in the yield. Accordingly, in order to reduce the basal plane dislocations which are carried over into the epitaxial film from the substrate, the use of a substrate having a small off-angle may be attempted. However, the current techniques may be limited to the use of an about 4°-off substrate.
In this circumstances, various processes have heretofore been studied so as to reduce the basal plane dislocations which are carried over into the epitaxial film from the substrate. For example, there have been reported the following processes:
a process where an epitaxial film is grown on an SiC single crystal substrate having a smooth surface, while controlling the relationship between the surface roughness of the epitaxial film to be grown, and the growth rate of the epitaxial film based on a predetermined conditional equation (please refer to Patent document 1);
a process where two kinds of buffer layers having different donor concentrations are previously grown on an SiC single crystal substrate, and an epitaxial film is grown through the buffer layers (please refer to Patent Document 2);
a process where the supply of the raw material gas is stopped for 5 to 30 minutes in the middle of the crystal growth step of growing an epitaxial film and the basal plane dislocation is converted into an edge location in the subsequent growth stage (please refer to Patent Document 3);
a process where a plurality of inhibiting layers having different nitrogen concentrations so as to suppress the basal plane dislocation density are provided on an SiC single crystal substrate, and an active layer composed of an SiC single crystal thin film is formed thereon (please refer to Patent Document 4); and
a process where a predetermined unevenness is formed on the surface of an SiC single crystal substrate to provide a physical wall in advance of the growth of an epitaxial film, and a basal plane dislocation is converted into an edge dislocation by collision against the wall during the growth of the epitaxial film (please refer to Patent Document 5).
In this connection, Patent Document 6 discloses that the substrate temperature is decreased by 50 to 100° C. on the way. However, this operation is performed when the introduction of the material gas is stopped in the middle of the crystal growth step so as to once interrupt the epitaxial growth.